中文摘要：

在讨论宽带RF—IF采样下变频器工作原理的基础上，详细地推导了5种典型的调制模式，并对采样下变频器本振信号的频率要求进行了论述。通过运算实例给出了采样频率fs及其有效位数的计算公式，结果证明：fs及其有效位数与射频带宽无关，只取决于中频的频谱分辨能力。

英文摘要：

Based on discussions on the operating principle of the broadband sampling down-converters, five typical modulation modes are deduced. Special requirement for the frequency selection of the local signals in the sampling down-converters is also given. The calculating formula of the sampling frequency fs and its significant digit are presented through computation examples. Computational results show that fs and its significant digit are not related to the RF bandwidth, and only determined by the resolution of the IF spectrum.